What to A/B Test?

I am looking to pitch a proposal to a prospective client on using Google Optimize for A/B testing. With the proposal, I also want to give them few A/B Test ideas on their website. However, I am not sure what ideas should I give behind what hypothesis. I read somewhere on the internet that your A/B Test hypothesis should go like this "If (we change this) then (we will get this) and the rationale is (this)". So my question is should I find out the rationale first before thinking on "if we change this".

I have quite a few ideas behind "if we change this" but shouldn't that come after I have the rationale. How do you come up with a hypothesis?

What to A/B Test?

First,You have to analyze user behaviors on website.For analyzing you can use some tools that have video recording and heatmaps.Then you can decide which elements needs to improve and create the hypotesis among these.